The studies in this Section have led us to the hypothesis that the energy state of one region of the structure of proteins would be coupled with those of distant regions to generate extra stabilizing energy. Thus, to investigate whether the strength of antigen binding is related to the reduction of the fluctuation of distant regions of the antibody by such coupling, we have developed 5 hybridoma cell lines, 4.74.6, 4.128.6, 4.145.10, 2.96.12 and 2.34.19 producing monoclonal IgG1 directed against yeast iso-1-cytochrome c. Monoclonal IgG1 4.74.6, 4.128.6 or 4.145.10 reacts with yeast cytochrome c but not with cytochrome c of Candida krusei, tuna, chicken, pigeon, rat, sheep, dog, porcine, bovine or horse. Monoclonal IgG1 2.96.12 or 2.34.19 reacts with both yeast and C. krusei cytochromes c. The former IgG1, but not the latter also reacts with tuna, rabbit and chicken cytochromes c in a lesser extent. None of these five monoclonal antibodies reacts with the apocytochromes c tested (yeast, C. krusei and horse). Monoclonal IgG1 4.74.6, 4.128.6 or 4.145.10 does not react with heme- or apofragments of yeast cytochrome c but does react with the productive fragment complexes of yeast cytochrome c or even with the hybrid complexes containing the horse heme fragment and yeast apocytochrome c. On the basis of tests with hybrid complexes, the antigenic determinants for these three antibodies appear to be located in residues 59 to 108. Monoclonal IgG1 2.96.12 and 2.34.19 do not react with any of these complexes or hybrid complexes. The results indicate that all of these five monoclonal antibodies recognize the three-dimensional configuration of the antigenic determinant formed by folding of the native cytochrome c (and also the complexes in the three former cases). In addition, the three former antibodies more specifically recognize the side chains of the amino acids in the determinant than the two latter. However, the two latter antibodies discriminate between the native protein and the fragment complex but the three former do not.